Leukocytes express a large repertoire of adhesion molecules which play important roles in lymphocyte homing, in phagocytosis, and in the recruitment of leukocytes to sites of inflammation. Many of these cell- extracellular matrix and cell-cell adhesion molecules are members of the integrin family. Integrins are heterodimeric cell surface glycoproteins containing one alpha subunit and one beta subunit. Using the homology polymerase chain reaction method, we have identified a partial cDNA clone encoding a novel integrin alpha subunit, alpha-A, in lymphocytes. The proposed project aims to study the structure, expression, and function of alpha-A. First, the complete sequence of alpha-A will be determined by cDNA library screening. Second, Northern blot analysis will be used to quantitate alpha-A mRNA in T and B cells, in other leukocytes, and in non- leukocytes. The effects of T cell activation on alpha-A mRNA levels will also be examined. It has previously been shown that certain integrins are more highly expressed on lymphocytes found in certain organs, consistent with the hypothesis that these integrins are organ-specific homing receptors. In this proposal, in situ hybridization will be used to detect tissue-specific differences in lymphocyte alpha-A expression. Third, the alpha-A subunit protein will be identified using antisera raised against peptides modeled after the deduced alpha-A amino acid sequence. Beta subunits which associate with alpha-A will be identified in co- immunoprecipitation, immunodepletion, and Western blotting experiments. Finally, cell adhesion assays and affinity chromatography will be used to attempt to identify ligands of alpha-A-containing integrins. Potential ligands to be examined include a variety of extracellular matrix proteins and endothelial cell surface proteins. These studies are designed to be a first step toward the determination of the role of alpha-A-containing integrins in normal and pathological immune responses.